The mechanical transmission system, or drive train, of a wind turbine comprises a wind rotor, a gearbox (since the turbine's rotation speed does not usually match the generator's speed) and an electric generator. The drive train includes a low-speed shaft mounted between the wind rotor and the gearbox, and a high-speed shaft mounted between the gearbox and the generator. In addition, the drive train includes a mechanical brake, which function is blocking the turbine for maintenance operations and which usually contributes in case of emergency stops such as the one that could occur facing a pitch mechanism fault obstructing blade feathering. Stopping a wind turbine is one of the most critical operations because it implies the generation of elevated stress levels that directly affect components of the wind turbine.
The physical constitution of the mechanical brake comprises a disc that rotates with the transmission shaft and some brake calipers that apply friction on the disc when activated electrically, hydraulically or pneumatically.
One of the most relevant aspects in the design of the mechanical brake is its placement on the drive train, since it could either be installed on the low-speed shaft or on the high-speed shaft.
For reduced power turbines (approximately 1 MW or less power) the most suitable location for the mechanical brake is on the low-speed shaft, the location reflected in patents JP2004124771 (A) and NL8302191 (A). In any event, there are instances of mechanical brakes consisting of a single disc and installed on the high-speed shaft. The mechanical brakes formed by a single disc present the following problems: The amount of energy to dissipate is the kinetic energy of the rotor unit plus the mechanical work developed by the aerodynamic torque during braking (which is not insignificant when the blades are in the power position, since, if the pitch mechanism has failed, these cannot feather out). This amount of energy defines the volume of the brake disc, thus defining both the thickness and diameter of the disc.
The critical braking of maximum energy lasts for a certain duration, since the torque must be at a certain definite value to stop the machine. This energy converts to heat in the disc and raises its temperature. In order to maintain temperatures within acceptable limits (above which the system would overheat and fail), the disc volume must be a certain value, hence its thickness and diameter could be increased. Having an elevated disc thickness would not help much since the heat would generate on its surface. The time elapsed during braking is insufficient for heat transfer from the surface to the mid-plane of the disc. Thus, there is a disc temperature gradient that decreases towards the inside or mid-plane. There is therefore a practical limit on the disc thickness, beyond which there is no benefit of a significant reduction in the surface temperature when augmenting the thickness. Consequently, only the diameter of the disc remains as the ultimate variable to increase the volume of the ferrous material and achieve reasonable surface temperatures on the disc/pad. With a single disc, the diameter of the brake for a wind turbine with an approximately 1 MW power rating would, as such, interfere with the adjacent elements at this position in the nacelle (namely the gear motors of the yaw) and the wear and maintenance on this element would prove to be excessively costly.
With the purpose to gain in ferrous material mass where store dissipated energy on braking during pitch mechanism fault preventing blade feathering, a mechanical brake has been conceived with at least two brake discs.